Spot meter & ZS vs. BTZS & incident light

noseoil · Jan 10, 2005

The most recent shot I posted to the standard gallery was of some metal tanks in full sun. With a spot meter, I was able to determine the highlight values and shadow values. How would an incident meter function in this situation for use with BTZS? Pointing it at the lens in full sunlight would completely miss the reflections from metallic surfaces, would it not?

This is still somewhat of a problem for me to understand. How does an incident meter deal with specular reflections? Do you just turn around and point it at the reflection of the light? If so, this is reflected light, not incident light. If not, how do you measure a bright reflection with an incident meter? This is an ongoing issue I have with trying to understand exposure and development using BTZS concepts.

Donald Miller · Jan 10, 2005

noseoil said:
The most recent shot I posted to the standard gallery was of some metal tanks in full sun. With a spot meter, I was able to determine the highlight values and shadow values. How would an incident meter function in this situation for use with BTZS? Pointing it at the lens in full sunlight would completely miss the reflections from metallic surfaces, would it not?

This is still somewhat of a problem for me to understand. How does an incident meter deal with specular reflections? Do you just turn around and point it at the reflection of the light? If so, this is reflected light, not incident light. If not, how do you measure a bright reflection with an incident meter? This is an ongoing issue I have with trying to understand exposure and development using BTZS concepts.

By specular reflections I assume that you are addressing small regions of highlights that exceed the scale of the film, the paper or both.

Typically specular highlights will be relatively small in relation to the more important tonal representations. For that reason I would not concern myself with specular highlights whether that would be with a spot or an incident meter. The reason that I would not concern myself with these is that they would tend to skew the development indicated.

In BTZS metering the meter dome would be pointed toward the camera lens from both the sunlit and shaded positions. The incident reading from any of these positions would be representative of a five stop scene brightness range. If the scene brightness ratio exceeds seven then the development is reduced. If the scene brightness ratio were less then seven then the development is expanded.

If you are speaking of larger areas of reflective nature in which detail is desired then the scale of the materials will not be exceeded if the incident metering is done in the manner that I have described.

argentic · Jan 10, 2005

noseoil said:
How does an incident meter deal with specular reflections? Do you just turn around and point it at the reflection of the light? If so, this is reflected light, not incident light. If not, how do you measure a bright reflection with an incident meter? This is an ongoing issue I have with trying to understand exposure and development using BTZS concepts.

AFAIK it doesn't deal with reflections. Since very bright reflection usually become blank in print you don't have to deal with them ?

mark · Jan 10, 2005

I hope this does not turn into a one is better than the other.

Noeoil-
I looked at your print and I would say that those really bright areas were not specular. That was just where the sun was hitting the tanks so you could meter from just that point. I always assumed specular meant the sun shining off Chrome. Seems the spot meter did a fine job.

wm blunt · Jan 10, 2005

Noseoil,
For that kind of situation I would take my lower(shade) reading as usually but for the high reading I would turn the dome of the meter slightly towards the bright metal and this would show a higher SBR and in turn would reduce dev. to make sure the high values weren't blocked. I sometimes do this if there is a bright value in the scene, such as clouds or white water. Hope this make sense.

Helen B · Jan 10, 2005

I don't see how incident metering can be used for metering specular reflections of light sources, or for light emitting objects/surfaces for that matter - both very similar situations. There are times when you want detail in such circumstances: backlit snow and aluminium-painted rooftops come to mind immediately, in addition to the sort of subject in noseoil's picture.

The use of an incident meter to determine correct exposure relies on the assumption that the illuminated surfaces in the picture have a reflectance factor that lies within a certain range of values. The reflectance factor in the lobe of specular reflection can exceed that range, and so experience and judgement, or a reflected-light meter, come in handy.

Best,
Helen

Claire Senft · Jan 10, 2005

BTZS Incident system

The BTZS incident system is very easy to use. It is based on the fact that from the blackest black to the whitest white that can be created with paint is approx. 41/2 to five stops difference in reflectivity when evenly lit. Film speeds are tested as outlined in the BTZS system. A shadow reading in the most important area is taken at 2 times the tested speed. The dfference in stops between highlite and shadow areas determines the developing time. Specular reflections are ignored since it is assumed that they should print white. The system assume that all tones from white to black are present in both highlighted and shadow areas. If they are not present then you can either accept that fact or change exposure/development time to compensate. It is an easily applied system that does not require a great deal of practice. To make the system work properly, one must preform some sensimetric tests...readings with a desitometer are required. Charting is also required but they can be hand drafted. One must consult the BTSZ manual since the subject can not be fully covered here.

I think the pros and cons, at least for me, is that on does not specifically know which zone a part of the subject will end up on with the incident system thereby making visulization more difficult. Problems that a spotmeter can cause...such as sensitivity differences by color, flare problems and judgement regarding readingsof distant items, IR light sensitivity etc ...are eliminated. In the best world one would use both systems. The testing is the same and the charting requirements similar.

noseoil · Jan 10, 2005

Thank you all for your comments and input. I'm working on the BTZS text as a method of understanding light better and consequently, exposure and development. Based on the above comments, I guess my trouble with understanding the BTZS concept, in this instance, stems from the lack of a uniform scale of values which fit the "normal range" for a shot. I added a variable which is not a natural part of most scenes, shiny metal.

Granted, this is an extreme example of reflections and perhaps my lack of proper wording is to blame for my use of the term specular reflections. I consider specular reflections to be the light reflected from the sun (or any light source) off of an object which has a reflective surface such as chrome, a mirror or shiny metal, for example. I suppose a highly reflective paint could also qualify if the finish is polished enough.

If I had exposed the shot I took while ignoring the reflections completely, the tanks would have been completely white over a large portion of their surfaces. I chose to reduce development and add exposure in order to try to balance the amount of light reflecting from the tanks and make it seem less blinding, while still retaining the feeling of reflected light. I use this tank farm as a test scene, as usually there isn't as much harsh light and contrast to deal with in the natural world.

So it seems that I should retain a balance between both worlds. The ZS and BTZS both have their place, I just need to keep plugging away at the learning end of things and continue shooting. Again, thank you all for your answers. tim

Helen B · Jan 17, 2005

'...a specular reflection is of the same intensity as the source...'

Um, Jay, I hope that you don't mind me offering a different interpretation:

Most materials reflect some of the light striking them in a specular manner and some in a diffuse manner. Few materials are totally specular or totally diffuse (there's a name for totally diffuse like 'La...ian' - Laplace, Lagrange, Lambert? - I forget: I'll have to look it up). Further: the specular part of the reflection often occurs in a lobe (ie a 'single ray' of incident light is reflected into a narrow cone of specular reflection. So when you see the reflection of the sun on most surfaces you can see it over a small range of angles, and thus it is visible as a bright spot that is larger than the one you would see from a mirror.

All that means that there is no need for the specular reflection to have anything like the same intensity as the source.

It's a lot easier to explain with a little intensity vs angle polar diagram, so I'll draw one and scan one in if you like.

'so an incident reading in direct sunlight should be the same as a spot reading of a specular reflection'

Oops, that doesn't follow. An incident reading in full sun is, and should be, very different from a spot reading of the sun*! The fact that a spot reading of the specular reflection is nowhere near a spot reading of the sun (unless you are reading the reflection of the sun off a mirror) is just an example of the specular reflection not being the same intensity as the source.

Best,
Helen

*not that one would ever contemplate doing such a thing.

Claire Senft · Jan 17, 2005

A matter of definition

My idea of a specular highlight is direct sunlight hitting polished chrome or other metals or glass and it is intended to be printed paper white in the print. The zone system or BTZS incident methods are intended to cover the range of tones from jet black in shadow to titianium white in direct sunlight. This is more than adequate for a full range print going from maximum black to paper base white. Another good example would be the intense scintillations on sidelit or backlit water that are in direct sunlight. If you tried to print them other than a textureless white with any system I believe it would be very difficult to do and would end up with a very strange looking photo. What would one expect to do with something that reads zone 27 for a brilliant, intense specular highlight(s) for example when you have skin tone that you wish to have at say zone 6 1/2?.

Helen B · Jan 17, 2005

There are, for sure, different interpretations of 'specular', especially when applied in different situations. My previous post was about reflecting surfaces ('specular reflections'), and Claire's post is about print values ('specular highlights') - '...a specular highlight is direct sunlight hitting polished chrome or other metals or glass and it is intended to be printed paper white in the print...'

For Claire's definition, with which I have no quarrel, there is indeed no reason to meter for the 'specular highlights'.

But don't we need some way to describe the bright, directional mirror-like reflections from non-mirror surfaces when we want them to record detail? The practical importance of the apparently academic distinction is a result of the assumptions made about the limits to the range of surface reflective index when using an incident meter. 'Specular' reflections may lie outside this range, and we may wish to record detail in them. If we wish to record detail in them an incident meter cannot, on its own, indicate the exposure required because the ratio of incident to reflected light is unknown.

How does that sound?

Best,
Helen

PS. "Lambertian". That's the name of a totally diffuse surface with equal intensity reflections over a hemisphere.

rbarker · Jan 17, 2005

Helen B said:
. . . But don't we need some way to describe the bright, directional mirror-like reflections from non-mirror surfaces when we want them to record detail? The practical importance of the apparently academic distinction is a result of the assumptions made about the limits to the range of surface reflective index when using an incident meter. 'Specular' reflections may lie outside this range, and we may wish to record detail in them. If we wish to record detail in them an incident meter cannot, on its own, indicate the exposure required because the ratio of incident to reflected light is unknown.

How does that sound?

Yes, I think so, and sounds great.

I think part of the issue here is the technique used in taking the incident reading. The usually-suggested method of pointing the incident dome at the camera tends to under-measure the intensity of light that creates the specular reflections, whereas pointing the incident dome at the light source (i.e. metering the light itself) takes that into account. It's rather like being aware of the solar disc in your peripheral vision versus looking directly at the sun.

The practical problem is that if the light source is the sun, the under-measurement resulting from the usual incident reading technique is extreme, and the light intensity is such that the highlight on film tends to spread/bleed into the surrounding area of the negative due, I believe, to failure of the anti-halation backing. As the print has an absolute maximum white value, the medium can't deal with the excess negative density, so all that is seen is the highlight and its bleed into the surrounding areas.

Although I haven't studied/tried the BTZS metering method, it seems to me that in situations (similar to that Noseoil confronted) that fall outside the usual range anticipated by the method, an additional meter reading needs to be added to the decision matrix - either a spot reading of the highlight, or an icident reading with the dome pointed at the light source (e.g. the sun in this case). Then, how one balances the exposure/development probably depends on which end of the luminence range one wishes to give precedence.

Helen B · Jan 17, 2005

I suggest doing a comparison between the reading given by an incident meter pointed directly at the sun, and a spot meter reading of the specular reflection from an aluminium-painted roof, for example. As soon as the sun comes out I'll do it.

Best,
Helen

Kirk Keyes · Jan 17, 2005

jdef said:
a specular reflection is of the same intensity as the source, so an incident reading in direct sunlight should be the same as a spot reading of a specular reflection. Jay

Not it you have the light integrating sphere on your meter.

If the dome is still on the incident meter, then you have just attenuated the light striking the light meter sensor.

If you removed the dome, then you have just turned your incident meter into a reflection meter. Some meters allow you to do this - but you need to have special reflectance attachments to get accurate numbers and also to know the field of view.

Kirk Keyes · Jan 17, 2005

Claire Senft said:
My idea of a specular highlight is direct sunlight hitting polished chrome or other metals or glass and it is intended to be printed paper white in the print.

Don't forget reflections of the sun on water are commons source of specular as well.

(edited) Ooops, I see you did get around to mentioning this.

noseoil · Jan 17, 2005

Helen, it sounds like you have a very well thought out approach to an interesting problem.

My concern would be pointing any type of meter directly at the sun. I'm not sure if the circuitry involved in any meter would be able to deal with a gain of this magnitude. Reflected sunlight from a roof sounds ok, but I would hate to see you ruin a good meter on a purely academic question. It seems to me that metering an incident specular reflection and then an incident value of full sunlight would make more sense than trying to deal with the sun's full illumination.

Since we (typically) don't take pictures of the sun itself, an incident sunlight reading and a specular reading would suffice.

In any event, please post your findings as I'm curious about the results. tim

Claire Senft · Jan 17, 2005

My final thought

The BTSZ incident system is not even intended to deal with any SPECIFIC reflectivity. If you read the manual on it and try it I firmly believe that you will find reflections that should show detail are accomodated very handily by the system. What may be difficult for one steeped in traditional zone system lore is the fact that you are not trying to place any object on any zone. You are merely providing an exposure and developing time to take care of all the tone the film can handle ..if they exist in the scene or not is immaterial. Everything from jet black in shadow to extreme white in sunshine is covered. This is a very firmly grounded system based on sensitometry. Read the manual, think about it and try it. It works very nicely.

Helen B · Jan 17, 2005

'Since we (typically) don't take pictures of the sun itself, an incident sunlight reading and a specular reading would suffice.'

Tim,

That's what I intended to do, but may not have made it clear! I wouldn't point a spotmeter at the sun.

Best,
Helen

Ole · Jan 18, 2005

Helen B said:
'Since we (typically) don't take pictures of the sun itself, an incident sunlight reading and a specular reading would suffice.'

Tim,

That's what I intended to do, but may not have made it clear! I wouldn't point a spotmeter at the sun.

Best,
Helen

APX 100, Lee IR filter, 4 x 1/2 second at f:22, developed in Windisch' Compensating Pyrocatechin:

Exposure was pure guesswork, the filter almost opaque, and the developer chosen for the extreme compensation. And then they stopped making APX100 sheet film...

Helen B · Jan 21, 2005

OK, the sun came out, but there were no aluminium-painted roofs handy, so I had to make do with wet blacktop.

Here are the readings to interpret as you wish:
Incident meter directed straight at the sun: f/11½ (no surprise!)
Spot meter reading off wet blacktop: f/64½

No matter which way I pointed the incident meter, the highest reading I could get was f/11½.

Meter set at EI 100, 1/125 second. Clear sky, slight haze. 2 pm, New York.

Best,
Helen

mark · Jan 21, 2005

SO you have one hell of a difference there. I hope someone chimes in and interprets this. I have never seen a difference between incident and sopt that was that large. I usually get a difference of maybe two stops.

DeanC · Jan 21, 2005

Helen B said:
OK, the sun came out, but there were no aluminium-painted roofs handy, so I had to make do with wet blacktop.

Here are the readings to interpret as you wish:
Incident meter directed straight at the sun: f/11½ (no surprise!)
Spot meter reading off wet blacktop: f/64½

No matter which way I pointed the incident meter, the highest reading I could get was f/11½.

Meter set at EI 100, 1/125 second. Clear sky, slight haze. 2 pm, New York.

Best,
Helen

Joining the thread late so apologies if I missed something above that makes my answer not make sense...

The above results are what I would expect, the spot meter reading of the highlights is about 2.5 stops over the incident reading. The incident meter is giving you the middle of the assumed 5 stop range (so, zone 5-ish) the spot meter of the wet pavement is giving you a highlight reading (zone 7-8 or so) about 2.5 stops above the average.

The spot meter is trying to tell you how bright the thing you're pointing it at is. The incident is trying to tell you how strong the lighting its in is. Similar ideas but different enough for 2.5 stops.

Dean

Helen B · Jan 21, 2005

Dean,

Er... 2½ stops from f/11½ to f/64½?

noseoil · Jan 21, 2005

Helen, I think he's trying to say there is a difference on the high end of 2 1/2 stops. I counted this one on my fingers (my calculator, the original "Plam") and I still get a 5 stop difference between the readings.

This is why I prefer to use a spot meter in these situations. That, and I don't own an incident meter. tim

P.S. Thanks for taking these readings for us.

mark · Jan 21, 2005

Tim

Both of helen's readings were the high values. Not the low and the high. She has no shadow values, if I read the post right.

I am going to assume she measured with the incident meter right next to the road facing the sun and spotted the same area.

Spot meter & ZS vs. BTZS & incident light

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